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Perhaps the most unusual single-seat fighter developed by Curtiss was the Model 70, which was designed from the outset to be flown either as a monoplane or as a biplane with the minimum of structural change. A metal semi-monocoque aircraft with fabric-covered wings, it was ordered on 23 November 1932 as the XF13C powered by a Wright SGR-1510-2 two-row radial rated at 600hp at 3050m. The designation XF13C-1 was assigned to it for test in monoplane form and XF13C-2 in biplane (or more strictly, sesquiplane) form. It was initially flown in December 1933 as a biplane, flying in monoplane form on 7 January 1934, and being delivered to the US Navy as the XF13C-1 on the following 10 February. Featuring a manually-operated retractable undercarriage, an enclosed cockpit, retractable upper wing leading-edge slats and trailingedge flaps, the XF13C-1 did not revert to biplane standard, but, in February 1935, was returned to Curtiss for various modifications, including installation of an XR-1510-12 engine affording 700hp at 2135m. With these changes it was redesignated XF13C-3. In biplane configuration, the XF13C was 30km/h slower than as a monoplane, but possessed a shorter take-off run and better low-speed characteristics. Trials were terminated in October 1935, primarily owing to lack of engine spares.